1. Technical Field
The present invention relates to a semiconductor device, and more particularly, to a DC offset correction circuit for canceling a DC offset included in an input signal, and a receiving system having the DC offset correction circuit.
2. Discussion of the Related Art
As semiconductor devices are made smaller and more integrated, different systems, for example, a transmitter and a receiver, are integrated in a single system like a multiband transceiver on one chip. A superheterodyne receiver, which is generally used, amplifies an input signal by always converting the input signal to the same intermediate frequency (IF) regardless of the frequency of the input signal. While having excellent performance and being easily embodied, the superheterodyne receiver includes a plurality of filters, amplifiers, and other circuits for the several time conversions of the frequency of the input signal. Thus, the configuration of a system with a superheterodyne receiver is complicated and the manufacturing cost thereof is high.
As a result, demand for a direct conversion receiver that transmits data without the conversion to IF is increasing. The direct conversion receiver decodes an input signal directly from a high frequency signal that is received thereby without having to perform a frequency conversion, unlike the superheterodyne receiver.
Compared to the superheterodyne receiver, since the number of parts of the direct conversion receiver may be reduced, the direction conversion receiver may have features such as high integration and low power consumption. In addition, since the structure of the direct conversion receiver is not complex, the cost of manufacturing a one-chip system therewith may be reduced.
However, the direct conversion receiver's oscillation frequency may drift, its frequency selection can be compromised, and it may create a DC offset. Accordingly, a receiving sensitivity of a direct conversion receiver is deteriorated. In particular, since the DC offset distorts the original input signal to be decoded, even though the distorted signal is received and decoded, the original signal may not be obtained.
Thus, the direct conversion receiver uses a DC offset correction (DCOC) circuit to cancel a DC offset included in the input signal. In general, a digital-to-analog converter (DAC) and a high pass filter are used as the DCOC circuit.
Although a DAC occupies a small area, it can be inadequate for a system, for example, Wideband Code Division Multiple Access (WCDMA), that receives in real time or continuously a signal having a DC offset that may frequently change according to a receiving environment.
Thus, a system like WCDMA uses a high pass filter to cancel the DC offset included in the input signal. However, although the high pass filter may cancel in real time or continuously the DC offset included in the input signal, it occupies a large area. In addition, the high pass filter cuts off not only a signal having no frequency, for example, a DC signal, but also a signal having a low frequency, for example, a low frequency AC signal. Thus, the amplitude of the input signal is attenuated and the input signal may be distorted.
Thus, since a DC or a low frequency signal includes information meaningful for modulation, a high pass filter can be designed such that a cut-off frequency, for example, −3 dB frequency, is close to DC so as to not deteriorate the signal's quality. However, since the high pass filter requires a higher resistance and capacitance to achieve the −3 dB frequency, the high pass filter may occupy an even larger area on a chip. Consequently, there is a need for a DCOC circuit having a small layout and that can cancel in real time a DC offset included in an input signal.